Talking about tryndamere in jungle, not in lane, in lane ofc he can still win, in jungle he gets very low (especially if he gets double golems) and if the other jungler can finish camps faster and ambush him then they win. Most have no idea what they are doing.
Counters to Cass? a good kennen, brand, and a good ahri, really if you out play her then you win, or any of these + a sion or someone with a stun, renekton can close and beat her up pretty easy too. Cass relies heavily on landing that first q.
Shyvanna is probably the fastest and safest threes jungler, no mana + tanky and aoe. Lee sin has been falling off for a few weeks now imo, still strong but not the free win he was.
Skarner I need to see to believe, I know he can do well in jungle, has less mana issues than other mana based junglers, but I don't think his levels 1-5 are strong enough as a jungler. Ganking at 6 is a pretty long wait in threes.